The goal of the proposed research is to elucidate the mechanisms by which avian oncornavirus proteins and RNA are assembled into virions. In particular, the role of proteolytic cleavage in this process will be investigated. Previous work has shown that the four major internal proteins are cleaved from a common precursor polypeptide. Purification of the protease responsible for cleavage will be attempted, and its specificity characterized. Experiments will be carried out to determine at what stage in virus maturation the precursor is cleaved, and what amino acid sequences are present at the cleavage sites. The possible localization of the precursor in intracellular previral particles will be examined. Many of the experiments will rely on immune precipitation of infected cell extracts to recover intracellular viral proteins. Further efforts will also be made to establish how general is the finding that some mammalian cells transformed with avian RNA tumor viruses do not cleave the precursor polypeptide. The nature of this block in cleavage will be investigated. In parallel to these studies on intracellular viral proteins, the structure of intact virions will be probed by means of reversible cross linking agents.